The invention relates to suspending agents of the hydroxyproyl methyl cellulose type and to a process for preparing vinyl chloride polymers by suspension polymerization of vinyl chloride wherein hydroxypropyl methyl cellulose ethers are used as suspending agents. Furthermore, it relates to new hydroxypropyl methyl cellulose ethers.
Hydroxypropyl methyl cellulose ethers have been known for a long time. A process for preparing them is for example described in U.S. Pat. No. 2,949,452.
Hydroxypropyl methyl cellulose ethers which have a methoxyl substitution of from 27% to 30% and a hydroxypropoxyl substitution of from 4% to 7.5% and hydroxypropyl methyl cellulose ethers which have a methoxyl substitution of from 28% to 30% and a hydroxypropoxyl substitution of from 7% to 12% as well as hydroxypropyl methyl cellulose ethers which have a methoxyl substitution of from 19% to 24% and a hydroxypropoxyl substitution of from 4% to 12% are commercially available. All these cellulose ethers can be used as primary suspending agents for the suspension polymerization of vinyl chloride. They are useful for controlling the size of the produced polyvinyl chloride polymer particles. Unfortunately however, for some applications the porosity of the resulting polymer particles and accordingly their ability to absorb plasticizers is not sufficient.
From European Patent Application No. 0 080 580 it is known to use a hydroxypropyl methyl cellulose having a methoxyl substitution of 20% to 32%, a hydroxypropoxyl substitution of 2% to 10% and a viscosity of 30 to 70 mPa's as a suspending agent for producing vinyl chloride (co)polymers which polymers are used for decreasing the viscosity of pasty polyvinyl chloride dispersions. Pastes of low viscosity are useful for producing articles which comprise a low amount of plasticizers. However, these suspending agents are not useful for producing vinyl chloride polymers or copolymers with high porosity.
The present invention provides new suspending agents of the hydroxypropyl methyl cellulose type for suspension polymerization of vinyl chloride which do not have the deficiencies of the known suspending agents of this type. Particularly, the present invention provides new suspending agents of the hydroxypropyl methyl cellulose type for suspension polymerization of vinyl chloride which suspending agents are useful for increasing or controlling the porosity of the produced polyvinyl chloride particles.
From German Democratic Republic patent specification No. DD 160354 a process for producing polyvinyl chloride by suspension polymerization of vinyl chloride is known. Combinations of oil-soluble fatty acid esters are used as suspending agents. It is an object of said patent to provide polyvinyl chloride which is able to absorb large amounts of plasticizer. However, it is not clear from the teaching what are the ranges of percentage of the methoxyl and hydroxypropoxyl substitution. The only hydroxypropyl methyl cellulose ether which the patent implies to have a relatively high methoxyl and hydroxypropoxyl substitution is stated to have a methoxyl degree of substitution of 1.95 and a hydroxypropyl molar substitution of 0.42.
Hydroxypropyl methyl cellulose ethers which have a methoxyl substitution of from 16.5% to 20% and a hydroxypropoxyl substitution of from 23% to 32% as well as hydroxypropyl methyl cellulose ethers which have a methoxyl substitution of from 19% to 28% and a hydroxypropoxyl substitution of form 7.5% to 11.5% are commercially available. These hydroxypropyl methyl cellulose ethers are known to be useful in emulsion paints.